Five bad alternator symptoms
An alternator’s main job is to keep up with the electrical demands being placed on the battery while driving. That’s an incredibly important point to keep in mind, so let’s read that again. Notice I didn’t say that the alternator’s primary job is to recharge the battery. An alternator’s main job is to keep up with the electrical demands being placed on the battery while driving. Although the alternator is designed to recharge the battery in the event you use more power from the battery than the alternator can provide, the alternator was never designed to recharge a completely dead battery. If you use your alternator to recharge a dead battery after a jump start, for example, you should know that you’re shortening the life of your alternator. If your battery is dead, recharge it with a real battery charger, not your alternator. Once you damage your alternator by using it as a battery charger, there’s no turning back. An new alternator costs around $495. A battery charger costs less than $100. So here are the symptoms of a bad alternator.
#1 Symptom of a bad alternator—Battery or charging warning light is lit
When the charging or battery light is lit, chances are your alternator or the voltage regulator portion of the charging circuit has bit the dust. Start by performing a full battery test and then an alternator test. See this post for helpful information on battery terminal cleaning and alternator testing.
#2 Symptom of a bad alternator—slow cranking when starting
The alternator has failed to keep up with the electrical demands while driving and the excess electrical demands came from the battery, which is now discharged. Slow cranking can also be caused by an aging battery that has built up too much sulphation. Or, the slow cranking can be caused by a parasitic battery drain due to a computer module that fails to go into “sleep” mode and continues to draw a high current level overnight. See this post for information on battery drain.
So even though slow cranking can be a bad alternator symptom, you should not automatically replace the alternator. Start by cleaning the battery terminals, performing a parasitic battery drain test and an alternator test before replacing anything.
#3 Symptom of a bad alternator—Dim lights at idle
Remember how the alternator’s job is to keep up with the electrical demands placed on the battery? Well, headlights and dash lights only consume about 15-20-amps and that’s well within the alternator’s ability to supply at idle speeds. However, if you’re at a stop sign and notice your headlights are dim, and you’re not running any other electrical accessories, you should suspect a bad alternator (after you’ve checked the integrity of the battery and alternator terminal connections). In other words any working and properly sized alternator should be able to supply 15-amps at idle.
However, an alternator may not be able to supply enough amperage at idle to keep up with the electrical demands of seat heaters, rear window defogger, heater blower on high AND headlights all running at the same time. If your headlights dim in that scenario, it’s not your alternator’s fault. So don’t confuse dim headlights with no other electrical loads with dim headlights when everything else is running.
In fact, if you’re at a stop and running all those accessories, you’re actually discharging your battery at a high rate. Take a look at what these electrical accessories draw
Rear Window defogger 25-amps, Electric AIR Pump 25-amps, Heated Seats 5-amps per seat, Headlamps (high) 20-amps, Blower Motor (High) 20-amps, Headlamps (low) 15-amps, Brake Lights 6-amps, Windshield Wipers 6-amps, Ignition 6-amps
Let’s say it’s winter and you’re at a stop light. You’re running your rear defogger, heated driver’s seat, wipers, headlights, blower on high and your foot is on the brake pedal. You’re consuming around 83-amps.
At an engine idle speed of around 750-RPMS, your alternator can only output about 35% of its rated output. So your 165-amp alternator can only provide around 58-amps, resulting in a 25-amp draw from the battery and a voltage drop that will dim your headlights. In this case, dim headlights would be considered NORMAL
#4 Symptom of a bad alternator—Grinding or whining noise
Alternators are driven by a belt. If the belt isn’t at the proper tension or the belt ribs are worn, it will slip around the alternator pulley, making a grinding, whining or chirping noise. However, alternators also contain bearings than can wear out prematurely from excessive heat or tension. A grinding noise is often the sign of a failing bearing. To check for bearing noise, follow the tips in this post
#5 Symptom of a bad alternator—Burning smell
As alternator bearings fail they produce resistance and that resistance can cause the drive belt to slip, overheat and burn resulting in a burnt rubber smell. An overheated alternator can also deteriorate the insulation on the stator and rotor windings, which also causes a burning smell.
If you choose an alternator that has approximately double the rated output of what your
system requires, then you are essentially oversizing your alternator. For example, most on highway vehicles have about 65 to 85 amps of electrical loads turned on when operating.
While an alternator may be rated for a specific output, it runs most efficiently when it’s
operating at roughly 35 to 50 percent of that rated output. A more efficient alternator
always decreases your fuel consumption.
Better Battery Life
Another added benefit of oversizing the alternator is the positive impact it can have on your
batteries. You’re able to reduce the depth of a battery’s discharge—also known as cycling.
That’s because an oversized alternator produces more output at lower speeds, including
idle. That increased output prevents the battery from having to help supply the load—and
this keeps it at a more ideal state of charge. The battery cycles less, which translates into
better battery life. With three to four batteries per vehicle, you can see how extending
battery life can be a big benefit.